Vertical-cavity, surface-emitting semiconductor lasers have been an object of research for many years. They offer high packing density, and batch processing of them on a wafer scale can lead to significant cost reductions for laser fabrication. Their very small optical cavities offer a level of low-power operation that was achieved with edge-emitting lasers only after years of research. The research on vertical-cavity, surface-emitting lasers intensified with the fabrication, as disclosed by Jewell et al. in U.S. Pat. No. 4,949,350, of a working array of such lasers having threshold currents in the milliamp range. In summary, they grew a laterally unpatterned vertical optical-cavity structure of two semiconductor interference mirrors separated by the lasing wavelength. The growth also formed midway between the mirrors an active quantum-well region that emitted at this wavelength. They then defined the individual lasers in the array by ion-beam etching through the entire vertical structures so as to form free-standing pillars. Each pillar constituted a separate laser. It had a height of 5.5 .mu.m and a diameter in the range of micrometers. The lasers were individually contacted on their tops while a conducting semiconductor substrate served as a common counter-electrode. Laser light was emitted through the substrate.
It was early recognized that the free standing, high aspect-ratio pillars created difficulties for practical laser arrays. Making electrical contact to the top of a 1 .mu.m pillar extending 5.5 .mu.m above the substrate cannot be done with normal semiconductor processing techniques. Jewell et al. suggested surrounding the pillar with a low-index material, such as a polyimide. Recent work by others have pursued this polyimide planarization, but the results have not been completely satisfactory. The polymeric polyimide cannot be completely cured at the thickness required for tall pillars. The soft planarizing polyimide serves as a support for after deposited metallizations and causes difficulty for the spot welding of contact wires to the metallizations since the excessive heat causes the polymer to melt and flow.
Many researchers, including Jewell et al., have suggested regrowing III-V materials around the pillars. To date, the regrown structures have suffered various disadvantages. Arnot et al. have described regrowth around quantum-dot pillars in "Photoluminescence Studies of overgrown GaAs/AlGaAs MOCVD and MBE Quantum Dots," Quantum Well for Optics and Optoelectronics, 1989 Technical Digest Series, volume 10, conference edition, pages 83-87. Their pillars contained multiple quantum wells, which were defined by a polymeric mask which was removed before regrowth. They measured the luminescence before and after regrowth. Their results showed degradation in the luminescence after regrowth for the smaller pillars, whether the regrowth was by molecular beam epitaxy (MBE) or organo-metallic chemical vapor deposition (OMCVD). However, planarization was achieved. Izrael et al. have described similar experiments in "Microfabrication andoptical study of reactive ion etched InGaAsP/InP and GaAs/GaAlAs quantum wires," Applied Physics Letters, volume 56, 1990, pages 830-832. Their quantum wire structures were defined by reactive ion etching (RIE) using overlying aluminum or nickel masks. The metallic masks were removed after etching and prior to regrowth by OMCVD. The regrowth significantly reduced edge recombination; however, they did not attempt to planarize. Planarization of the structure was not achieved. The disparity in degradation between these two results may be caused by both of the structures being exposed to ambient air between the etching and the regrowth.
Lebens et al. have disclosed the fabrication of quantum-dot structures by selective area growth in "Application of selective epitaxy to fabrication of nanometer scale wire and dot structures," Applied Physics Letters, volume 56, 1990, pages 2642-2644. The dot pattern was etched into a Si.sub.3 N.sub.4 mask overlying an AlGaAs substrate. Thereafter, a 100 nm thick GaAs layer was grown by OMCVD. However, the GaAs selectively grew only in the opening of the Si.sub.3 N.sub.4 mask, thereby forming quantum dots.
Tokumitsu et al. disclose a similar process with organo-metallic molecular beam epitaxy (OMMBE) in "Molecular beam epitaxial growth of GaAs using trimethylgallium as a Ga source," Journal of Applied Physics, volume 55, 1984, pages 3163-3165. OMMBE uses MBE for some of its components but uses OMCVD for others. Thus, it ultimately relies on chemical vapor deposition (CVD) and the concomitant chemical reaction at the growing surface. Tokumitsu et al. used SiO.sub.x as the masking layer. After regrowth in the openings of the SiO.sub.x mask, the mask was lifted off.
The use of low molecular-weight SiO.sub.x and Si.sub.3 N.sub.4 masks is disadvantageous for fabricating the vertical-cavity, surface-emitting lasers for which ion-beam etching is required to form the high aspect-ratio vertical features.
In contrast to OMCVD and OMMBE, MBE is a purely ballistic process of co-depositing multiple species from respective sources. Therefore, MBE is much less sensitive to selective growth than OMCVD. Harbison et al. disclose an attempt to extend selective area regrowth to MBE in "Tungsten patterning as a technique for selective area III-V MBE growth," Journal of Vacuum Science and Technology B, volume 3, 1985, pages 743-745. They patterned a metallic mask on a GaAs substrate, similarly to Lebens et al., but the mask was composed of tungsten. Then they grew a GaAs layer. The GaAs grew epitaxially in the opening of the mask but grew as a polycrystallite above the tungsten. The tungsten was then lifted off, thus removing the polycrystalline GaAs. Harbison et al. noted that a fluorine plasma etch removes the tungsten while leaving the III-V material but a chlorine etch removes the III-V material while leaving the tungsten. MBE regrowth is more limited for pillars because of the directionality and non-selectivity of the growth. That is, it is difficult to obtain uniform growth on the sidewalls of high aspect-ratio (tall) structures. Furthermore, although MBE provides exceedingly fine thickness control, it is a very slow growth process.